The present invention is generally related to improved robotic devices and methods, particularly for telesurgery.
Minimally invasive medical techniques are aimed at reducing the amount of extraneous tissue which is damaged during diagnostic or surgical procedures, thereby reducing patient recovery time, discomfort, and deleterious side effects. Many surgeries are performed each year in the United States. A significant amount of these surgeries can potentially be performed in a minimally invasive manner. However, only a relatively small percentage of surgeries currently use these techniques due to limitations in minimally invasive surgical instruments and techniques and the additional surgical training required to master them.
Advances in minimally invasive surgical technology could dramatically increase the number of surgeries performed in a minimally invasive manner. The average length of a hospital stay for a standard surgery is significantly longer than the average length for the equivalent surgery performed in a minimally invasive surgical manner. Thus, the complete adoption of minimally invasive techniques could save millions of hospital days, and consequently millions of dollars annually in hospital residency costs alone. Patient recovery times, patient discomfort, surgical side effects, and time away from work are also reduced with minimally invasive surgery.
The most common form of minimally invasive surgery is endoscopy. Probably the most common form of endoscopy is laparoscopy which is minimally invasive inspection and surgery inside the abdominal cavity. In standard laparoscopic surgery, a patient""s abdomen is insufflated with gas, and cannula sleeves are passed through small (approximately xc2xd inch) incisions to provide entry ports for laparoscopic surgical instruments.
The laparoscopic surgical instruments generally include a laparoscope for viewing the surgical field, and working tools defining end effectors. Typical surgical end effectors include clamps, graspers, scissors, staplers, and needle holders, for example. The working tools are similar to those used in conventional (open) surgery, except that the working end or end effector of each tool is separated from its handle by, e.g., an approximately 12-inch long, extension tube.
To perform surgical procedures, the surgeon passes these working tools or instruments through the cannula sleeves to a required internal surgical site and manipulates them from outside the abdomen by sliding them in and out through the cannula sleeves, rotating them in the cannula sleeves, levering (i.e., pivoting) the instruments against the abdominal wall and actuating end effectors on the distal ends of the instruments from outside the abdomen. The instruments pivot around centers defined by the incisions which extend through muscles of the abdominal wall. The surgeon monitors the procedure by means of a television monitor which displays an image of the surgical site via a laparoscopic camera. The laparoscopic camera is also introduced through the abdominal wall and into the surgical site. Similar endoscopic techniques are employed in, e.g., arthroscopy, retroperitoneoscopy, pelviscopy, nephroscopy, cystoscopy, cisternoscopy, sinoscopy, hysteroscopy, urethroscopy, and the like.
There are many disadvantages relating to current minimally invasive surgical (MIS) technology. For example, existing MIS instruments deny the surgeon the flexibility of tool placement found in open surgery. Most current laparoscopic tools have rigid shafts and difficulty is experienced in approaching the worksite through the small incision. Additionally, the length and construction of many endoscopic instruments reduces the surgeon""s ability to feel forces exerted by tissues and organs on the end effector of the associated tool. The lack of dexterity and sensitivity of endoscopic tools is a major impediment to the expansion of minimally invasive surgery.
Minimally invasive telesurgical systems for use in surgery are being developed to increase a surgeon""s dexterity as well as to allow a surgeon to operate on a patient from a remote location. Telesurgery is a general term for surgical systems where the surgeon uses some form of remote control, e.g., a servomechanism or the like, to manipulate surgical instrument movements rather than directly holding and moving the tools by hand. In such a telesurgery system, the surgeon is typically provided with an image of the surgical site at the remote location. While viewing typically a three-dimensional image of the surgical site on a suitable viewer or display, the surgeon performs the surgical procedures on the patient by manipulating master control devices at the remote location, which control the motion of servomechanically operated instruments.
The servomechanism used for telesurgery will often accept input from two master controllers (one for each of the surgeon""s hands), and may include two robotic arms. Operative communication between master control and an associated arm and instrument is achieved through a control system. The control system typically includes at least one processor which relays input commands from a master controller to an associated arm and instrument and from the arm and instrument assembly to the associated master controller in the case of, e.g., force feedback.
One objective of the present invention is to provide improved surgical techniques. Another objective is to provide improved robotic devices, systems, and methods. More specifically, it is an object of this invention to provide a method of compensating for friction in a minimally invasive surgical apparatus. It is a further object of the invention to provide a control system incorporating such a method of compensating for friction.
The present invention provides improved devices, systems, and methods for compensating for friction within powered automatic systems, particularly for telesurgery and other telepresence applications. The invention allows uninhibited manipulation of complex linkages, enhancing the precision and dexterity with which jointed structures can be moved. This enhanced precision is particularly advantageous when applied to the robotic surgical systems now being developed. The friction compensation systems of the present invention address static friction (typically by applying a continuous load in the direction of movement of a joint) and the often more problematic static friction (generally by applying alternating loads in positive and negative joint actuation directions). The invention can accommodate imprecise velocity measurements by applying an oscillating load whenever the joint velocity reading falls within a low velocity range. Preferably, the oscillating load is insufficient to move the joint without additional input, and significantly reduces the break away input required to initiate movement. In the exemplary embodiment, a duty cycle of the oscillating load varies, favoring the apparent direction of movement of a velocity reading. The amplitude of the duty cycle may also vary, typically increasing as the velocity reading approaches zero.
In a first aspect, the invention provides a method of compensating for friction in an apparatus. The apparatus has at least one component that is selectively moveable in a positive component direction, and in a negative component direction. An actuator is operatively connected to the component. The method includes obtaining a component velocity reading, and defining a velocity reading region extending between a selected negative velocity reading and a selected positive velocity reading. A duty cycle is generated so that the duty cycle has a distribution between a positive duty cycle magnitude (corresponding to a friction compensation force in the positive component direction) and a negative duty cycle magnitude (corresponding to a friction compensation force in the negative component direction). The distribution is determined by the component velocity reading when it is within the velocity reading region. The actuator is loaded with a load defined by the duty cycle signal.
Preferably, the duty cycle signal will have a continuous positive duty cycle magnitude (which corresponds to the friction compensation force in the positive direction) when the component velocity reading is greater than the selected positive velocity reading. Similarly, the duty cycle signal will have a continuous negative duty cycle magnitude (corresponding to the friction compensation force in the negative component direction) when the component velocity reading is less than the selected negative velocity reading.
In the exemplary embodiment, the distribution of the duty cycle between the positive and negative magnitudes is proportional to the component velocity reading positioned within the velocity reading region. The positive and negative duty cycle magnitudes may take a gravity compensation model into account. Such a gravity compensation model may determine a variable gravity compensation force to applied to the component, for example, to artificially balance an unbalanced linkage system. Such a gravity compensated system may further benefit from a determination of a frictional compensation force corresponding to the gravity compensation force in both the positive and negative directions. In other words, in addition to compensating for friction, the method of the present invention may accommodate compensation factors for both friction and gravity, thereby simulating or approximating a friction-free balanced system, significantly enhancing the dexterity of movement which can be accommodated.
The selection of an appropriate oscillating frequency can significantly enhance friction compensation provided by these methods and systems. Hence, the frequency will preferably be selected so as to be sufficiently slow to enable the actuator (often including an electrical motor and a transmission system such as gears, cables, or the like) to respond to the directing duty cycle signal by applying the desired load, and sufficiently rapid so that the load cannot actually be felt, for example, by physically moving the joint and varying a position of an input master control device held by a surgeon. In other words, the frequency is preferably greater than the mechanical time constraints of the system, yet less than the electrical time constants of an electrical motor used as an actuator. Preferred duty cycle frequency ranges of the exemplary telesurgical system described herein are in a range from about 40 Hz to about 70 Hz, preferably being in a range from about 50 Hz to about 60 Hz. Application of these oscillating loads can facilitate movement of a joint in either a positive or negative direction, particularly when the velocity reading is so low that the system cannot accurately determine whether the system is at rest, moving slowing in a positive direction, or moving slowly in a negative direction. Once velocity measurement readings are high enough (a given measurement reading accuracies) in a positive or negative direction, a continuous (though not necessarily constant) force in the desired direction can overcome the dynamic friction of the joint.
In yet another aspect, the invention provides a method comprising manipulating an input device of a robotic system with a hand of an operator. An end effector is moved in sympathy with the manipulating step using a servomechanism of the robotic system. A velocity reading is obtained from a joint of the robotic system. An oscillating friction compensation load is applied on the joint when the velocity reading is within a first reading range.
Preferably, a continuous friction compensation load is applied when the reading is within a second reading range, typically above (either in the positive or negative direction) a minimum value. The continuous load can compensate for friction of the joint, and may vary so as to compensate for gravity when an orientation of the joint changes. The oscillating load similarly compensates for static friction of the joint in the positive and negative directions, at varying points along the load oscillation duty cycle. This method is particularly advantageous for compensating for friction and/or gravity in a joint of the input device for the robotic system, particularly where the oscillating load is less than a static friction of the joint so that the end effector can remain stationary in the hand of the operator.
In another aspect, the invention provides a telesurgery method comprising directing a surgical procedure by moving an input device of a telesurgery system with a hand of an operator. Tissue is manipulated by moving a surgical end effector in sympathy with the input device using a servomechanism of the telesurgery system. Static friction is compensated for in at least one joint of the robotic system by applying an oscillating load to the at least one joint when an absolute value of a velocity reading from the at least one joint is less than a velocity reading error range.
While the friction compensated joint may support the surgical end effector, it will preferably support the input device. The oscillating load is generally effected by applying a duty cycle to an actuator, and preferably by altering the duty cycle in response to the velocity reading so as to facilitate movement of the joint towards the positive orientation when the velocity reading is positive, and toward the negative orientation when the velocity reading is negative.
In yet another aspect, the invention provides a telepresence system comprising a master including an input device supported by a driven joint. A slave includes an end effector supported by a driven joint. A controller couples the master to the slave. The controller directs the end effector to move in sympathy with the input device. A sensor operatively associated with at least one of the driven joints generates a velocity reading. An actuator drivingly engages the at least one driven joint. The actuator applies an oscillating load on the joint to compensate for static friction of the joint when the velocity reading is within a low velocity range.
Preferably, the oscillating load is insufficient to move the at least one driven joint when the master remains stationary. In the exemplary embodiment, the end effector comprises a surgical end effector, and the slave is adapted to manipulate the surgical end effector within an internal surgical site through a minimally invasive surgical access.